* Stable: This is a common antonym, referring to someone who is emotionally steady and predictable.
* Consistent: This emphasizes reliability and predictability in someone's behavior or character.
* Steady: This implies a calm and composed nature, the opposite of someone who is volatile and unpredictable.
* Constant: This suggests a lack of change or fluctuation in mood or behavior.
* Unchanging: This refers to someone who is unchanging and predictable.
* Predictable: This is a more general antonym that simply means someone who is easy to understand and anticipate.
The best antonym will depend on the specific context in which you are using the word "mercurial."